Gas turbines often include a compressor, a number of combustors, and a turbine. Typically, the compressor and the turbine are aligned along a common axis, and the combustors are positioned between the compressor and the turbine in a circular array about the common axis. In operation, the compressor creates compressed air, which is supplied to the combustors. The combustors combust the compressed air with fuel to generate hot combustion products, which are supplied to the turbine. The turbine extracts energy from the hot combustion products to drive a load.
To increase efficiency, modern combustors are operated at temperatures that are high enough to impair the combustor structure and to generate pollutants such as nitrous oxides (NOx). These risks are mitigated by directing the compressed air over the combustor exterior, which cools the combustor, before premixing the air with fuel to form an air-fuel mixture, which generates lower levels of NOx when combusted.
For these reasons, the combustor typically includes a flow sleeve that defines an annular passageway about the combustor. The annular passageway receives air from the compressor through a diffuser positioned adjacent to the combustor. The air impinges against the transition duct and combustion liner for cooling purposes. The air then travels in a reverse direction through the annular passageway toward the combustion cap assembly, which houses the fuel nozzles. A portion of the air is also diverted from the annular passageway to cool the cap assembly.
For example, an end face of the cap assembly is exposed to high temperatures of the combustion chamber. Thus, the end face is normally cooled with air diverted from the annular passageway through openings in the cap assembly wall. The diverted air impinges against and passes through the end face into the combustion chamber. Thus, the diverted air is not pre-mixed with fuel, which exacerbates NOx generation.
The air traveling through the annular passageway experiences pressure loses. Due to these pressure losses, an increased amount of air is needed to cool the cap assembly, resulting in a lower percentage of premixed air in the combustor. Also, the air flow pressure through the end face may not be sufficient to overcome a dynamic pressure wave that is present in the combustion chamber due to flame instability. The dynamic pressure wave may exert a pressure on the end face that impedes or stops the cooling flow, causing the end face to heat and potentially fail.
Supplying higher pressure air to the cap assembly would reduce the amount of air needed for cooling, so that a relatively larger percentage of the combustion air could be premixed with the fuel, reducing NOx generation. Further, supplying higher pressure air would improve the dynamics barrier. Thus, a need exists for supplying higher pressure air to the head end of the combustor, such as to the cap assembly.